It is widely accepted that activation of NMDA-type glutamate receptors (NMDARs) is required for the strengthening and weakening of cortical synaptic connections during visual development. NMDAR dysfunction has been implicated in a wide variety of neurological disease states such as epilepsy, schizophrenia, stroke, and neuropathic pain. Most research has focused on the role of NMDARs postsynaptically. Recently it has been found that neurons in the visual cortex also express NMDARs that can modify presynaptic functions (preNMDARs). PreNMDARs are expressed early in cortical development and modulate the release of neurotransmitter through mechanisms such as spike-timing dependent long-term depression. Studies have also implicated preNMDARs in disease states such as epilepsy. Despite this knowledge about preNMDARs, it is not clear what allows for their function and developmental regulation. Unlike postsynaptic NMDARs, preNMDARs in the developing neocortex do not require the coincident binding of glutamate and depolarization to be active. Instead, they are tonically active and therefore able to increase the release of neurotransmitter in the absence of depolarization. PreNMDARs are also sharply downregulated in the neocortex following postnatal day 20. I hypothesize that a NMDAR subunit endows preNMDARs with the ability to be tonically active and is essential for their developmental regulation. I will therefore determine: 1) The NMDAR subunit composition of preNMDARs early in life which allows for their tonic activity, 2) The mechanism by which preNMDAR subunits allow for tonic activity, and 3) If these subunits are required for timing-dependent long- term depression and for preNMDARs to modulate action potential-driven release. To accomplish this, I will use electrophysiology, electron microscopy, and biochemical fractionation on mice which lack specific NMDAR subunits. These studies will demonstrate the molecular composition of preNMDARs and the mechanism by which they function. Therefore, these experiments are expected to provide new molecular targets for therapies to treat epilepsy, stroke, neuropathic pain, and schizophrenia and may clarify the mechanism by which current NMDAR-mediated therapies act.